The Blaugrana legend told So Foot: “He will need some time. Barca is like a final exam for a footballer. It is like Dembele is passing a Master's degree right now because not everyone can play for this club.

“Why? Because you have to know things three times more than elsewhere. Barca play on barely 30 metres of the pitch.

“Dembele has a lot of talent, he is very fast, but here, he is not going to have the space he had at Dortmund or Rennes. He had more space, so more time there.”

Pressed on what Dembele must do to fit the Barca mould, Xavi added: “He will have to learn to think faster, in a few thousandths of a second. This is where we will see if he has the mentality.

“He must say to himself: ‘l am a Barca player’. You have to be mentally strong, to have conviction.

“There are average players who spent 15 years at Barca, because they had character. And there are some excellent players who did not do anything because they couldn't handle pressure.

“In training, when you saw them, you would say to yourself: ‘They will break everything. They're going to be legends’. But no. As soon as they entered the pitch, their legs started shaking, they did not want the ball anymore. And you would wonder: ‘Holy f***, what's happening to them?’”

Xavi considers mental strength to be the most important quality for any player at the top of the game, with Dembele only needing to look at those around him at Barcelona and those across the Clasico divide with Real Madrid for inspiration.

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The World Cup winner added: “Mental strength is what stabilises the performance. That's what makes you stronger.

“When there is fire, Marcelo, [Luka] Modric or Sergio Ramos do not hide. On the contrary, it's at this moment that they appear.

“What did Lucas Vazquez do against PSG? He entered the pitch with the desire to win. A missile. The guy even went to [Presnel] Kimpembe. And there you say to yourself: ‘What is he doing? He's crazy or what?’ No, he's just mentally strong.”